This study will examine the impact of repeated daily exposure to selected indoor air pollutants on the developing lung in the juvenile ferret. The hypothesis to be tested is that the ferret will be a better model for the developing human lung than any presently-available species, except perhaps for primates. The ferret lung resembles that of the human more than do the lungs of rats or mice, and also the ferret undergoes very rapid growth from neonate to adult size in about 20 weeks. The rationale for the experimental plan is that (1) concentrations of indoor air pollutants (e.g., nitrogen dioxide, NO2) are often much greater than outdoors, (2) young children are repeatedly exposed to these pollutants during their growth and development, (3) increased evidence of respiratory disease in children is associated with high levels of indoor air pollutants, (4) animal experiments suggest that chronic exposure during lung development may produce lasting detrimental effects. The experimental approach will be to expose neonate ferrets daily to moderate levels of NO2 or a model particulate until they reach adult size. Specific experiments will address effects on lung morphology, tissue permeability, particle clearance capability, cell damage nad repair, and lung macrophage activity. In some experiments, animals will be sacrificed at intervals during the repeated exposure to observe progressive effects of the treatment. In other experiments, the lung properties. The long-term goal is to obtain information on effects of chronic low level pollutant exposure in the lung in order to provide guidelines for minimizing the risks to humans.